1. Field of the Invention
The invention relates to abrasive blasting techniques and, more particularly, to apparatus and methods for generating a pressurized stream having abrasive particles.
2. Description of the Prior Art
Abrasive blasting has been used successfully to polish, etch, abrade, cut, drill, clean or otherwise process a variety of materials. Abrasive blasting typically involves a process in which a fluid and abrasive particles are blended together to form an abrasive mixture. This abrasive mixture is then sent through a nozzle to direct a pressurized stream, containing abrasive particles, at the surface of an object. Conventional abrasive blasting apparatus have been used in a variety of applications ranging from the removal of rust from bridges to the restoration of oil paintings.
High-performance abrasive blasting machines have been developed that produce an accurate abrasive stream using pressurized air and fine powders formed from such materials as crushed glass, silicone carbide and aluminum oxide. These high-performance machines often display great versatility; they can possess sufficient power to cut and drill materials as hard as diamonds as well as the precision to delicately clean debris from fragile items like fabric and paper. High-performance blasting apparatus typically include a chamber in which high-pressure air and a very fine powder are mixed to form an abrasive mixture. The chamber feeds the abrasive mixture to a small, precisely machined nozzle which dispenses the mixture as an accurately shaped, narrow airstream in which the fine powder particles move at relatively high speeds. Achieving the full potential and versatility of precision abrasive blasting machines, however, requires the proper application of a number of key variables, including: air pressure, powder flow rate, nozzle size, type of powder, nozzle distance to a workpiece, and nozzle angle with respect to the workpiece.
Specifically, the air pressure at which an abrasive mixture is fed to a nozzle is directly related to the velocity of the abrasive particles striking a workpiece. The higher the air pressure, the faster the particles move and the greater the cutting speed is. The lower the air pressure, the slower the particles and the lower the cutting speed.
The number of abrasive particles exiting a nozzle per unit of time is referred to as the "particle flow rate". Generally, the greater the particle flow rate, the greater the cutting action. In order to achieve better cutting action with higher particle flow rates, it is often necessary to also increase the air pressure of the abrasive mixture to maintain particle velocity, which tends to decrease as airstream mass increases. Cutting action can also decrease quickly at very high particle flow rates due to turbulence caused by, for example, interference between the particles bouncing off the workpiece and those coming out of the nozzle. At extremely high air pressures and flow rates, turbulence can also be created within the nozzle, which can further slow the particles and decrease the cutting action.
The nozzle size and shape normally determine the area that will be impacted by the abrasive particles. The larger a nozzle exit area is, the greater the impact area is. Also, the distance and angle between a nozzle tip and the workpiece play a large part in determining the area covered as well as the cutting rate.
The type of abrasive particles is also an important variable in a typical high-precision abrasive blast process. For instance, in order to achieve consistent, sputter-free, streamlined flow from a nozzle, it is usually necessary to use uniformly sized particles. When the particles vary in size, they tend to clump together as smaller particles fill in gaps between the larger particles, thereby adversely effecting the flow characteristics.
One of the most critical problems confronting designers of precision abrasive blasting methods and apparatus has been developing techniques for increasing the velocity of the particles while maintaining a highly accurate abrasive airstream. In that regard, it has been generally recognized that simply increasing the air pressure of an abrasive mixture to achieve an accurate airstream with greater particle speeds has its limitations. For instance, extreme air pressures can adversely effect an airstream by distorting its shape, size and flow as that airstream exits a nozzle. Also, interior walls and tips of conventional nozzles normally experience excessive wear when conveying abrasive particles traveling at relatively high velocities. Consequently, those concerned with the development of high-precision abrasive blasting equipment have recognized the need for improved techniques of increasing the speed of abrasive particles while maintaining an accurate abrasive stream and avoiding undue nozzle wear.